Fragment of the support structure of an engine - Grey cast iron - Modern Times

Marianne. Senn (EMPA, Dübendorf, Zurich, Switzerland) & Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

Stratigraphic representation: none

Fig. 4: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation can be compared to Fig. 10.

Analyses performed:
Metallography (nital etched), Vickers hardness testing, LA-ICP-MS, SEM/EDX.

The remaining metal is a high P and Si grey cast iron with elevated Mn and V contents (Tables 1 and 2). The structure contains black graphite flakes and graphite nodules (Fig. 4) as well as angular grey manganese sulphide inclusions which can contain a dark alumina-rich centre (Fig. 4 and Table 1). The graphite flakes are irregular and vary in size. At low magnification the flakes are evenly spread over the entire surface with a tendency to form clusters. Some porosity is noticeable. Under SEM, in the BSD-mode, an additional eutectic phase is visible (Fig. 5). After etching one can see how the graphite is surrounded by alpha-iron in a pearlite matrix (Figs. 6 and 7). The lamellar pearlite includes steadite (Fe3P) and MnS inclusions. According to the cast iron diagram after Maurer (Bargel and Schulze 2008, 257), the structure is typical for a hypoeutectic pearlitic grey cast iron (C content <4.3 mass%, Si content ca. 2.0 mass%) including pearlite, steadite and graphite. The slow cooling rate and the higher Si level have favoured the formation of graphite. The high P content favours the growth of interconnected networks of steadite. The average hardness of the metal is HV1 160. This hardness is only approximate. Normally cast iron hardness is determined with a Brinell test, but due to the small size of the sample this could not be carried out. The calculated HB is about 150.

 

Elements Ni/Co Al P Ti V Cr Mn Co Ni Cu As Mo Ag Sn Sb W C* mass%
Median mg/kg 2.9 < 20000 1200 2000 700 4500 160 460 110 610 20 < 10 20 < <4.3
Detection limit mg/kg - 4 73 8 1 11 2 1 3 1 2 3 1 0.4 1 2 -
RSD % 2 - 47 26 38 19 18 3 2 10 14 30 - 17 13 - -

*visually estimated

Table 1: Chemical composition of the metal. Method of analysis: LA-ICP-MS, Lab Inorganic Chemistry, ETH.

 

Elements

O Al Si P S Ti Mn Fe Total
MnS (centre) 23 21 20 < < 5.7 30 2.3 102
MnS < < < < 35 < 60 2.0 98
Metal (average of 5 similar analyses) < < 2.0 2.4 < < 0.8 95 101
Steadite (Fe3P) (average of 3 similar analyses) < < < 16 < < 1.1 88 105

Table 2: Chemical composition (mass %) of the metal. Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

The thin corrosion crust is limited to three sides of the sample and some remains of a paint coating (orange) are visible (Fig. 3). In bright field, the corrosion crust appears light-grey, under polarised light-orange (fig. 8 and 9). It is mainly composed of iron oxides (Table 3 and Fig. 10). The Pb and Ba-rich paint system has been applied directly onto the oxidized Fe, Si and Mn-rich casting skin (Fig. 11). The paint layer is probably an anodic inhibitor made from minium (red lead) with a barium sulphate filler. Because of the interference of Pb with S in the EDX spectra S is difficult to detect.

 

Elements

O Mg Si P S Mn Fe Ba Pb Total
Inner light-grey corrosion layer (Fig. 11) 34 < 14 4.5 < 7.5 47 < < 107
Inner light-grey corrosion layer (Fig. 11) 20 < < < < 1.3 76 < < 98
Dark-grey casting skin (Fig. 11) 27 < 14 < < 21 33 < < 95
Dark-grey casting skin (Fig. 11) 29 0.8 17 < < 20 29 < < 97
Paint system (Fig. 11) 20 < < < < < < 18 55 93
Grey corrosion layer (Fig. 10) 35 < 1.1 < 0.6 < 59 < < 97
Grey corrosion layer (Fig. 10) 36 < 1.4 < < 0.6 64 < < 102

Table 3: Chemical composition (mass %) of the corrosion layer (from Figs. 10 and 11). Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

Corrected stratigraphic representation: none

The metal is a hypoeutectic pearlitic grey cast iron with significant amounts of Si and P. A P-content in excess of 0.4 mass % causes a decrease in the tensile and impact strength. P is concentrated in the hard, brittle steadite phase. Supporting elements for machines are often manufactured from such metal because it can absorb a high degree of vibration. The metal is still covered by the casting skin which was left as a natural protection against corrosion. An applied Pb-based anodic paint system formed a further protection layer. The corrosion is minimal.

References on object and sample

References sample

1. Auskunftsblatt der Sammlung des Verkehrshauses der Schweiz Luzern (Wasserverkehr) zu VHS-404.

 

References sample

2. Senn, M. Prüfbericht 205’340-2, 2011.

References on analytic methods and interpretation

3. Bargel, H-J., Schulze, G. (ed.) (2008) Werkstoffkunde, Springer, 249-270.